That wasn’t included to answer a burning question for readers, or to give credit to the font designers. It was there so that, when we needed to correct a typographical error or update the text, the Production Department could quickly match the main type.
I see similar typographical lines in such new books as The Ghostwriter Secret, a Brixton Brothers mystery by Mac Barnett. The copyright page dutifully states:
The text for this book is set in Souvenir.And indeed the main text is in that font, with a contrasting display font for the chapter titles. But fonts are so much easier to come by these days that many authors and designers (in this case Lizzy Bromley) don’t stop at just one.
The Ghostwriter Secret also has passages supposed to replicate pages from a mid-20th-century series book, albeit one with slightly distorted capitals. And others meant to look like the products of three different typewriters, one of them missing the letter T. And a school permission slip, a young detective’s handwritten notes, and an author’s autograph—all set in different fonts.
The typewriter fonts are particularly interesting because they’re not actually monospaced. Unlike real typewriters, their spaces, periods, and Is are narrower than their Ws and Ms. In sum, those fonts are simply reminiscent of typewriters.
(I note that the current paperback of Mama Makes Up Her Mind, from Da Capo, also uses a typewriter-reminiscent font on its cover.)
And whom is that illusion meant for? Fewer and fewer young readers have much experience with typewriters. (I've made this point about “Typewriter Realism” before.) The ordinary, unpublished documents today’s kids see are rarely in that format. Their all-too-familiar-looking documents from an office or school is almost certainly be in Times, Helvetica or Arial, or even Comic Sans.